If they have responsive sites, this shouldn't change anything. Personally I find SEO way more challenging when they have a separate mobile site so the whopping majority of my clients have responsive sites.

Coupled with the new(ish) emphasis on AMP I think this is very interesting. I wonder if this move is indicative of a different ranking system. But like Joy said if their assets are web 4.0 then there shouldn't be any ripples from this. Thanks for sharing the news!

To echo what everyone else said, be sure to test new layouts/changes on all mobile devices and then desktop. Every time you crawl the site with Screaming Frog, make sure you set a mobile user agent for at least one of the crawls to make sure pages are being found. If the site is responsive, then it should be OK... but don't assume. Check everything.

Another big thing to watch is site speed. Google is has put a large emphasis on mobile for years, but it hasn't really hit mainstream until relatively recently. Be sure make sure both mobile and desktop are optimized for content performance, critical rendering path, and rendering performance.

Unlike their desktop counterparts, web applications do not require a separate installation process: enter the URL and we are up and running -- that’s a key feature of the web. However, to make this happen we often have to fetch dozens, and sometime hundreds, of various resources, all of which can add up to megabytes of data and must come together in hundreds of milliseconds to facilitate the instant web experience we are aiming for.

Yes, in many parts they're talking about developing web APPs, but the fundamentals carry over to all websites. Make sure your client has a quick loading site, optimize file delivery, compress images, and make sure the user gets a phenomenal experience.

Does this change anything? No. Google has been telling us to do these things for years, so it really shouldn't change all that much.